In virtually all tissues of every eukaryotic organism, polarized export of cell surface components is critical for a range of important cellular functions including protein secretion, assembly of the plasma membrane, cell signaling, cell polarization, cell migration, ciliogenesis, and cytokinesis. Defects in polarized export underlie or exacerbate a number of human diseases including cancer. We pioneered the genetic analysis of polarized export in yeast, identifying and characterizing a system of components that are all structurally and functionally conserved in higher eukaryotic organisms. Sec4p, the founding member of the rab GTPase family, acts as a master regulator by promoting the function of three different effectors: Myo2p a type V myosin that actively transports vesicles along polarized actin cables, the yeast homolog of the Lgl tumor suppressor Sro7p that physically links Sec4-GTP to the t-SNARE Sec9p, and the exocyst a complex of eight different gene products that interacts with components on the vesicle surface as well as polarity determinants on the cell cortex and thus tethers incoming secretory vesicles to these specialized sites. This proposal seeks to probe in greater detail the mechanism by which the exocyst acts to promote polarized export. We will define the role of the exocyst in the regulation of SNARE complex assembly and in the tethering of secretory vesicles to exocytic sites at the cell cortex. We will also critically test a model of exocyst function that requires a cycle of exocyst assembly and disassembly for ongoing vesicular traffic in vivo. These studies will shed light on one of the most poorly understood yet centrally important elements of the eukaryotic membrane traffic machinery.